The present invention relates in general to geogrids though there is some applicability to mesh structures in general. A geogrid is a grid whose primary purpose is to strengthen or reinforce soil and has open meshes into which soil particles can lock. If made by orienting a plastics starting material, the starting material would normally have a thickness greater than about 1, 1.5 or 2 mm. A geogrid is in effect made up of strands (also called ribs) which are interconnected at bars running across the geogrid in the TD or are interconnected at junctions (also called nodes or intersections), whether or not the strands are continuous throughout the geogrid as they would be in the case of say a woven geogrid. The thickness of a geogrid, as measured at the junction, would be greater than about 0.5 mm or 0.75 mm and may well be greater than about 1.00 mm or 1.5 mm or 2.0 mm. The mesh size (also called mesh opening size or aperture size) should if possible be capable of allowing the soil or aggregate particles to interact therewith and provide optimum anchoring or interlock.
Geogrids can be manufactured in many different ways, for instance by stitch bonding fabrics made of for instance polyester filaments and applying a flexible coating such as a PVC coating, or by weaving or by knitting, or even by spot-welding oriented plastic strands together. However, although not confined to such materials, the present invention is primarily concerned with geogrids which are formed by uniaxially or biaxially orienting a plastics sheet starting material which has been provided with holes. The holes form meshes in the product. In a uniax geogrid of this type, transverse bars are interconnected by strands. Biax geogrids of this type comprise oriented strands and junctions at which the strands meet, substantially each strand having each end connected to such a junction, whereby sets of parallel tensile members run through the geogrid, each tensile member being formed of a succession of substantially aligned strands and respective said junctions interconnecting the strands.
The present invention also relates to methods of making geogrids. In methods using a plastics sheet starting material which has been provided with holes, a stretch is applied to stretch out strand-forming zones between adjacent holes and form oriented strands from such zones, thereby providing a uniax geogrid. A stretch can be applied in a direction at right angles to the first stretch to stretch out other strand-forming zones between other adjacent holes and form oriented strands from the latter zones, whereby zones between groups of holes form junctions interconnecting the oriented strands and a biax geogrid is formed.
U.S. Pat. No. 4,374,798 and U.S. Pat. No. 5,053,264 disclose uniax and biax mesh structures of the general type with which the present invention is concerned, but it has now been appreciated that those mesh structures do not have great stability in the diagonal direction in that the mesh structures can be extended in the diagonal direction without great application of force due to parallelogram distortion of the mesh structure.
Uniax geogrids are extensively used where the stress is primarily in one direction, for instance when reinforcing embankments. In such structures, stresses are transferred from the soil along the strands and into the transverse (TD) bars which can be thicker than the strands and are anchored in the soil. Biax geogrids are extensively used in the reinforcement of granular layers in roads, parking areas, container storage areas and other hard standings. The strands extend in the MD and in the TD. As a consequence, the physical properties of the mesh structures are optimal in these two directions. However, it has been appreciated that there are weaknesses in the structure when tested between these two directions. Thus while the previous geogrids have high strength and stiffness in the longitudinal and transverse directions, it has now been appreciated that the loading from for example a heavy wheeled vehicle imposes radial stresses in the geogrid, i.e. stresses radiating in all directions from the loading zone.
It is desirable to provide more strength in directions other than the MD and TD without grossly reducing the strength of the mesh structure in at least one of the MD and TD.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.
Any discussion of the prior art throughout the specification is not an admission that such prior art is widely known or forms part of common general knowledge in the field.